Energy submetering system

ABSTRACT

A system for submetering artificial energy consumption in a multi-user environment in which a single central system source services the several separate users by means of individual space conditioning subsystems is disclosed which enables accurate proportioning of subsystem energy consumption by monitoring the time intervals of the OFF/ON status of each individual space conditioning subsystem along with the temperature of the conditioned space and utilizes the monitored time intervals and temperature in conjunction with the relative heat capacity of each individual conditioned space to determine the heat consumption of each subsystem relative to all the subsystems served by the central source.

BACKGROUND OF THE INVENTION Field of the Invention

The field of the invention relates generally to the field of energyusage metering and, more particularly, to a submetering system formulti-tenant or multi-owned buildings having individual spaceconditioning subsystems.

CROSS-REFERENCE TO RELATED APPLICATION

Reference is made to a related application Ser. No. 718,147 of J. H.Pejsa, the inventor in the present application, and T. H. Holland filedof even date and assigned to the same assignee as the presentapplication. By that invention energy consumption is submetered forconstant flow systems by monitoring the ON/OFF status of each individualspace conditioning subsystem, monitoring the temperature differentialacross each separate heat exchanger, and utilizing the monitored ON timeand the temperature differentials in conjunction with the relative heatcapacity of the measured fluid to determine the heat or coolantconsumption of each such subsystem relative to all such units.

In the present invention, on the other hand, submetering is accomplishedby monitoring the time intervals of the OFF/ON status of each individualspace conditioning subsystem along with the temperature of theconditioned space and utilizing the monitored time intervals andtemperature in conjunction with the relative heat capacity of eachindividual conditioned space to determine the heat consumption of eachsubsystem relative to all the subsystems served by the central source.

DESCRIPTION OF THE PRIOR ART

It has been customary to artificially heat and cool multi-tenant ormulti-owned buildings such as apartments, office buildings andcondominiums by employing large central systems to serve a plurality ofseparately leased areas. In this manner the central system suppliesheating or cooling energy in the form of steam or hot or chilled waterto individual heat exchangers or space conditioning subsystems locatedin each rental unit. This approach is quite efficient from thestandpoint of heating or cooling the entire building. The cost is, inturn, normally borne by the renters or owners as part of generalbuilding overhead apportioned without reference to actual energy use asby using unit square footage as a basis.

This approach has several serious drawbacks, however. It may beinherently unfair. For example, location has been found to have asignificant impact on annual average heating loads. In a cold climate, anorth facing apartment may require three to four times as much heatingenergy as one of equal square footage facing south. In addition, withsuch an approach there is very little incentive for conservation byindividual tenants because those who conserve still share the cost withthose who waste. As energy costs, in general, continue to rise, the needfor conservation is becoming greater and the concern of landlords,tenants and utility companies is increasing. This has produced a needfor a system that is both fair and one which encourages conservation.

One concept which provides incentives which lead to both lower overallusage and lower individual consumption is that of using directindividual consumption monitors or submetering systems. Several suchsystems exist for use with central steam or hot water heating systems,for example. They are conventional liquid flow meters together withtemperature sensors to measure actual energy supplied to individualrental units. However, such devices are quite expensive and requirepiping and other changes which entail substantial installation costs,particularly in the retrofit environment.

The scope of the problem is large. It has been estimated that there aremore than ten million residential units alone in buildings served bycentral steam or hot water systems and this number is likely to grow inthe next few years. It is apparent that a definite need exists for alow-cost, relatively accurate system for submetering energy consumption.

SUMMARY OF THE INVENTION

By means of the present invention, there is provided a low-costsubmetering system for hot or chilled water space-conditioning systemswhich monitors a plurality of individual energy deliveryspace-conditioning systems and which readily lends itself toretrofitting existing structures. The present invention measures energyconsumption by monitoring the state of each individual energy deliverysubsystem, i.e., the "OFF/ON" status and the temperature of theconditioned space.

The system includes a central receiving and monitoring station whichincludes a master transceiving unit having calculating, data storage anddisplay capabilities. Remote transceiver or store units are located ateach submetered space-conditioning subsystem. These units transmitsignals, normally converted to digital form by an analog to digital(A/D) converter, related to OFF/ON state and temperature data inresponse to interrogation by the master unit at desired time intervals(Δt). Communication between units may be by any convenient mode such as2-way power line carrier, telephone line, radio signal or directconnection.

In operation, at each preselected Δt, normally every few seconds, everyremote unit is sequentially interrogated by the master unit over acommunication channel as by 2-way powerline carrier. If the energydelivery system is ON, the remote unit responds with digital signalsrepresenting the ON state and the temperature and the temperature of theconditioned space. The master unit accumulates and integrates these dataindividually for each submetered unit and thereby monitors the fractionof the energy consumption of all units attributable to that unitaccording to the equation: ##EQU1## where F_(j) represents thefractional energy usage of a subsystem of interest, and S_(j) representsthe relative energy usage for any particular unit out of N units. Thisequation is developed in detail below. Of course, by factoring theappropriate conversion factor into the equation, energy share can becomputed as a proportion or as a customer cost in dollars rather than asQ or energy usage. A permanent record may also be kept if desired.

This method of submetering energy consumed in mastermetered buildings,while still an approximation, comes close to the accuracy ofconventional BTU meters or calorimeters and by eliminating the need fordirect measurement of flow, this method is significantly less costly.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a block diagram of one embodiment of the invention;

FIG. 2 is a graph of a typical time vs. temperature cyclicalrelationship for heating a conditioned space.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 depicts a block diagram of a typical submetering system utilizingthe invention. This includes a master unit 10 in 2-way communicationwith a plurality of remote units 11 as illustrated by lines 12 and 13.The master unit 10 is also connected to an output device illustrated at14. The master unit is one which is capable of continuously calculatingconsumption or energy usage and integrating such usage over a period oftime for each remote unit served. The master unit is normally programmedto interrogate each remote unit on a continuous basis at preselectedintervals. Such a device is normally microprocessor controlled and unitswhich will perform the task are readily available on the market and canbe programmed as necessary to the invention by one possessed of ordinaryskill in the art.

Each remote unit 11 is designed to measure and transmit the temperatureof the conditioned space and the ON or OFF status of the spaceconditioning subsystem upon interrogation by the master unit 10. Theseparameters may be sensed directly from a thermostat 15.

FIG. 2 is a graphical representation of typical cyclical temperatureversus time curve for a conditional space served by an individualspace-conditioning subsystem during the heating season. The cycle startswith the endpoint of the "ON" portion of the cycle at which time (t₀) tothe temperature of the conditioned space is at the desired set pointcontrol temperature (T₀). This, of course may include any override ifsuch undesirable characteristic be inherently present in the operationof the thermostatic control device used. During the initial timeinterval or OFF cycle, t₀ to t₁, the temperature decays at anexponential rate related to the heat loss characteristic to such asystem when make-up or replacement heat is zero. At time t₁ thetemperature of the conditioned space has fallen to a value T₁ and thethermostatic control device causes the space-conditioning subsystem toswitch again to the ON mode. The space-conditioning subsystem remains inthe ON mode until the space temperature again is equal to the set pointtemperature, i.e., T₂ =T₀ which occurs in the time interval t₁ to t₂which becomes t₀ for the next cycle and the process repeats.

The temperature decay during the OFF interval of the cycle can berepresented by the well-known relationship

    T=ae.sup.b·Δt                               (1)

where T is the absolute temperature, Δt is the decay time interval and aand b are constants which will now be developed with reference to FIG.2.

It is readily seen that at T=T₀, Δt=t₀ -t₀ =0 and at T=T₁ Δt=t₁ -t₀.Thus, at t₀, T=a=T₀ and

    T=T.sub.0 e.sup.bΔt                                  (2)

Since at t₁, T=T₁,

    T.sub.1 =T.sub.0 e.sup.b(t.sbsp.1.sup.-t.sbsp.0.sup.)      (3)

and ##EQU2## and, substituting in equation 2, we have at any time t;##EQU3##

The dashed lines for each cycle of FIG. 2 represent the continuation ofthe temperature decay to time t=t₂ which can be expressed as ##STR1##where T_(est) is the theoretical or estimated temperature at time t₂were there no heat added by the space conditioning subsystem.

The total energy delivered to the space-conditioning subsystem during anOFF/ON cycle can be given by

    Q=MC(T.sub.2 -T.sub.est)                                   (7)

or ##EQU4## where MC is the relative heat capacity of the submeteredconditioned space. It has been found that the area of the conditionedspace is proportioned to the heat capacity of that space in a particularbuilding environment. It thus becomes a good way to allocate theconstant MC such that for a rental space S_(j) ##EQU5## and theproportion of the energy used for all rental space because ##STR2##

In this manner the amount of energy required to heat or cool aparticular submetered conditioned space unit can be apportioned withsurprising accuracy.

Each remote unit may be connected so that it senses both the ON/OFFstates of the space conditioning subsystem and the temperature of theconditioned space by connection to the thermostatic device whichcontrols that space. This requires only a connection to sense the statusof the thermostat switch and a simple temperature sensor which in someapplications may be the thermostat temperature sensor itself. For mostapplications, each remote unit contains a conventional analog to digitalconversion system to convert the data assimilated to digital form fortransmission to the master unit.

In the normal set-up any particular application will involve a pluralityof the remote units 11 coordinated by and in communication with a masterunit 10. This give the system the ability to continually monitor energyusage by a relatively large number of units in a large building orcomplex.

In operation, the master unit will interrogate each of the remote unitson a predetermined time basis such as at an interval of every fewseconds depending on the number of units required to be addressed. Itwill then receive status and temperature data from each of the units.Any type of output means desired could be used to display energy usage,print billings, store data for future reference or other use.

The embodiments of the invention in which an exclusive property or rightis claimed are defined as follows:
 1. A method for submeteringartificial energy consumption in a multi-user environment in which acentral source services a plurality of individual users by means ofindividual space conditioning subsystems comprising the stepsof:monitoring the time interval of the OFF segment and the time intervalof the entire OFF/ON cycle for each cycle of each individual spaceconditioning subsystem; monitoring the temperature of each conditionedspace to determine temperature decay function during the OFF segment andthe actual space temperature at the conclusion of each OFF/ON cycle foreach cycle of each individual space conditioning means; determining thetotal artificial heating or cooling energy supplied by each suchindividual space conditioning subsystem during each complete OFF/ONcycle according to the relationship

    Q=MC(T.sub.2 -T.sub.est)

wherein:Q is the artificial heating or cooling energy; MC is therelative heat capacity of the conditioned space; T₂ is the conditionedspace temperature at the conclusion of the ON segment of the controlcycle; and T_(est) is the theoretical extrapolated decay temperature atthe end of the complete OFF/ON cycle; and utilizing the total artificialenergy supplied by each individual space conditioning subsystem todetermine the heat consumption of each such subsystem relative to allsuch subsystems served by said central source as a fraction of the totalthereof.
 2. A method according to claim 1 wherein the theoretical decaytemperature T_(est) is given by ##EQU6## wherein: T₀ is the conditionedspace temperature at the beginning of the OFF segment of the controlcycle;T₁ is the conditioned space temperature at the end of the OFFsegment of the control cycle; t₀ is the time at T₀ ; t₁ is the time atT, and t₂ is the time of the complete OFF/ON control cycle.
 3. Themethod of claim 1 wherein said relative heat capacity MC is a factorbased on the area of said individual conditioned space.
 4. The method ofclaim 3 wherein said relative heat capacity MC is a factor based on thearea of said individual conditioned space.
 5. A system for submeteringartificial energy consumption in a multi-user environment in which acentral source services a plurality of individual users by means ofindividual space conditioning subsystems comprising:a plurality ofremote monitoring units, one associated with each said spaceconditioning subsystem further comprisingmeans for monitoring the timeinterval of the OFF/ON status of each individual space conditioningsubsystem; means for monitoring the temperature of the conditionedspace; master control means for interrogating each of said plurality ofremote monitoring units receiving information therefrom, storing andprocessing said information and producing an output relative to theenergy consumption thereof; 2-way communication means for enablingcommunication between said master control units and said remote units;and output means in communication with said master control means forreproducing an output relative to said energy usage.